Certain water-thinnable coating compositions composed of acidified addition copolymers containing pendent carboxyl groups and pendent aminoester groups formed by aminoethylating pendent carboxyl groups with an excess of alkylenimine and epoxy resins are known as shown by U.S. Pat. No. 3,719,629 to Martin et al, issued Mar. 7, 1973. These acidified copolymers are cationic acid salts, i.e., the salt groups are provided by reacting basic groups with an acid. This creates a number of serious disadvantages. For example, because of the cationic nature of the copolymer, it cannot be readily formulated into a coating composition by blending it with the conventional anionic modifiers normally employed in coating compositions, acid sensitive pigments such as the carbonates cannot readily be employed, and the conventional anionic pigment dispersants cannot be utilized in the usual manner. Another disadvantage, due to the fact that the copolymer product is an acid salt, is that it cannot be satisfactorily employed as a coating material for unprimed metal substrates since such acid salts usually cause flash rusting.
Water-based coating compositions containing basic salts of polycarboxylic acid resins containing amine groups formed by iminating a portion of the carboxyl groups are shown by U.S. Pat. No. 3,494,847 issued to Yurcheson et al on Feb. 10, 1970. The coating compositions disclosed in U.S. Pat. No. 3,494,847 generally utilize as the polycarboxylic acid resin certain fatty acid adducts containing unsaturation for curing, but in some cases interpolymers of acrylic and other vinyl monomers, at least one of which contains a hydroxyl group, are contemplated. In such cases, the interpolymer is cured with an amine-aldehyde condensate to which may be added a polyepoxide.
U.S. Pat. No. 3,945,963, issued to Levine et al on Mar. 23, 1976, described emulsions of acrylic interpolymers in combination with certain resins derived from epoxides. These resins are either very high molecular weight (above 20,000) or defunctionalized (and thus not a polyepoxide) by reaction of the epoxy groups, such high molecular weight or defunctionalization being necessary in order to provide the one-package compositions contemplated.
In addition to the foregoing disadvantages, water-thinnable and water-based compositions of the type disclosed in the aforementioned patents possess additional disadvantages. Thus, compositions of the type described in U.S. Pat. No. 3,494,847 and 3,719,629 usually have a relatively short potlife, i.e., they react to a viscous, unusable state within a short time after the copolymer and epoxy resin components are mixed together. The potlife of such mixed compositions generally ranges from less than one hour to a few hours. Moreover, the amine group-containing copolymer components of the patented compositions are also usually deficient in stability, particularly when stored at elevated temperatures (e.g., 200.degree. F. or above), often becoming unusable in from less than one day to a few days.